Du Yuan

Poly(ethylene glycol) Conjugated Poly(lactide)-Based Polyelectrolytes: Synthesis and Formation of Stable Self-Assemblies Induced by Stereocomplexation

A series of pH-responsive amphiphilic poly(N,N-dimethylaminoethyl methacrylate)-block-poly(D-lactic acid)-block-poly(N,N-dimethylaminoethyl methacrylate) conjugated with poly(ethylene glycol) (D-PDLA-D@PEG) and D-PLLA-D@PEG copolymers were synthesized using a combination of ring-opening polymerization and atom-transfer radical polymerization followed by sequential quaternization of PDMAEMA chains and azide-alkyne click reaction with alkyne-end PEG. Gel permeation chromatography, nuclear magnetic resonance, and Fourier transform infrared spectroscopy results demonstrate the successful synthesis of the copolymers, and the conjugated PEG percentages in the copolymers can be tuned by the feeding ratios in the quaternization reaction. Conjugating PEG onto the PDMAEMA segments also successfully facilitated the D-PDLA-D@PEG, D-PLLA-D@PEG, and its corresponding 1:1 D/L mixtures to be dissolved directly in aqueous solution at the desired concentration range without using any organic solvents unlike the copolymers without PEG conjugation (D-PDLA-D and D-PLLA-D). We demonstrate control over micellar size, charge, and stability via three different preparation pathways, i.e., solution pH, percentages of PEG conjugation in the copolymers, and formation of PLA stereocomplex in micellar core. Static and dynamic light scattering studies demonstrate that the size of the core-shell micelles increases when the solution pH is reduced due to the protonation of the PDMAEMA segments that caused the osmotic pressure within the micelle to increase until the micelles reached a maximum size. It is interesting to note that the micelles formed by 1:1 D/L mixtures have larger swelling ratios as well as aggregation number and hydrodynamic radius that do not change significantly with pH and dilution, respectively, as compared to micelles formed from individual D or L forms of the copolymers. The enhanced stability of the pH-responsive micelles prepared by direct dissolution of the 1:1 D/L mixtures of the PEG conjugated PLA-based polyelectrolytes in aqueous medium is attributed to the stereocomplex formation between PLLA and PDLA in the micellar core as confirmed by wide-angle X-ray scattering measurements.

Facile Layer-by-Layer Self-Assembly toward Enantiomeric Poly(lactide) Stereocomplex Coated Magnetite Nanocarrier for Highly Tunable Drug Deliveries

A highly tunable nanoparticle (NP) system with multifunctionalities was developed as drug nanocarrier via a facile layer-by-layer (LbL) stereocomplex (SC) self-assembly of enantiomeric poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) in solution using silica-coated magnetite (Fe3O4@SiO2) as template. The poly(lactide) (PLA) SC coated NPs (Fe3O4@SiO2@-SC) were further endowed with different stimuli-responsiveness by controlling the outermost layer coatings with respective pH-sensitive poly(lactic acid)-poly(2-dimethylaminoethyl methacrylate) (PLA-D) and temperature-sensitive poly(lactic acid)-poly(N-isopropylacrylamide) (PLA-N) diblock copolymers to yield Fe3O4@SiO2@SC-D and Fe3O4@SiO2@SC-N NPs, respectively, while the superparamagnetic properties of Fe3O4 were maintained. TEM images show a clearly resolved core-shell structure with a silica layer and sequential PLA SC co/polymer coating layers in the respective NPs. The well-designed NPs possess a size distribution in a range of 220-270 nm and high magnetization of 70.8-72.1 emu/g [Fe3O4]. More importantly, a drug release study from the as-constructed stimuli-responsive NPs exhibited sustained release profiles and the rates of release can be tuned by variation of external environments. Further cytotoxicity and cell culture studies revealed that PLA SC coated NPs possessed good cell biocompatibility and the doxorubicin (DOX)-loaded NPs showed enhanced drug delivery efficiency toward MCF-7 cancer cells. Together with the strong magnetic sensitivity, the developed hybrid NPs demonstrate a great potential of control over the drug release at a targeted site. The developed coating method can be further optimized to finely tune the nanocarrier size and operating range of pHs and temperatures for in vivo applications.

Novel linear-dendritic-like amphiphilic copolymers: synthesis and self-assembly characteristics

Novel linear-dendritic-like poly(D or L-lactide)-b-poly(acrylic acid) (l-PD(L)LA-b-d-PAA) amphiphilic copolymers were synthesized for the first time via the combination of single-electron transfer living radical polymerization (SET-LRP), ring-opening polymerization (ROP) and thio–bromo “Click” chemistry. As a macroinitiator for initiating ROP of lactide, dendritic-like poly(tert-butyl acrylate) (PtBA) with a single focal hydroxyl group (OH-d-PtBA) was first synthesized via the “branch” and “growth”, followed by hydrolysis of the PtBA segment to yield the targeted linear-dendritic-like amphiphilic copolymers PL(D)LA-b-PAA. Light scattering and TEM studies revealed that the l-PDLA-b-d-PAA and l-PLLA-b-d-PAA copolymers self-assembled into vesicles in an aqueous environment, respectively. When mixed in a 1 to 1 ratio, the corresponding aggregates changed from vesicles to spherical micelles, possibly due to the formation of a more stable and dense packing of the hydrophobic PLA segments resulted from the stereocomplex between the enantiomeric PLA blocks. Meanwhile, the size of the formed aggregates changed with variation of the pH value, indicating that these aggregates possess the pH-dependent swelling and shrinking properties, which endow the formed aggregates with great potential for a controllable drug delivery system. This new type of amphiphilic block copolymers should find potential applications in the biomedical area due to their unique features.

Amphiphilic Conetworks and Gels Physically Cross-Linked via Stereocomplexation of Polylactide

Amphiphilic conetworks (APCNs), consisting of hydrophilic poly[poly(ethylene glycol) methyl ester acrylate] (PPEGMEA) and hydrophobic stereocomplex of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), were prepared by free radical copolymerization of PEGMEA with acrylate macromonomer of the PLA stereocomplex. The effects of stereocomplexation and the amount of PLA stereocomplex on the rheology properties of APCNs were investigated. The results indicated that the APCNs was stronger in the presence of stereocomplexation compared with the that of nonstereocomplex system, and the strength of the APCNs increased with the increasing of the amount of PLA stereocomplex. The storage modulus of the APCNs could be easily tuned from 1200 to 4300 Pa by incorporating 2-10% of stereocomplex PLA. On the other hand, the swelling behavior of APCNs decreased with the increasing content of hydrophobic PLA cross-linker.

β phase PVDF-hfp induced by mesoporous SiO2 nanorods: synthesis and formation mechanism

Poly(vinylidene fluoride) (PVDF) homopolymer and its copolymers in their β phases are the most promising polymeric materials for electroactive devices. In this work, a simple and effective strategy is developed to fabricate β-poly(vinylidene fluoride-co-hexafluoropropylene) (β-PVDF-hfp) with characteristic piezo-/ferro-electric properties using mesoporous SiO2 nanorods. The nanorods offer 2-fold arrangement for the –OH groups, namely, anisotropic rod shape and ordered mesopores. We report, for the first time in the PVDF system, the direct evidence of hydrogen bond interaction during β phase formation through XPS analysis. Our study further reveals that besides well-known facilitation by intermolecular interaction, the arrangement of surface functional groups is essential for the formation of β-PVDF-hfp. This proposed mechanism manages to explain the current controversy in the formation mechanism. We further demonstrate that our approach can be extended to a wider intermolecular interaction regime than hydrogen bonding, thus benefitting current development in PVDF homo/copolymer-based electroactive devices.

Polymer nanocomposite hydrogels exhibiting both dynamic restructuring and unusual adhesive properties.

Polymer nanocomposite (NC) hydrogels exhibiting both dynamic restructuring and unusual adhesive properties in wet and dry states have been prepared in an efficient and straightforward way via free radical polymerization of poly(ethylene glycol) methyl ether acrylate (PEG) in the presence of silane-modified sodium montmorillonite (NaMMT). The dynamic restructuring of the NC gel has been demonstrated by almost instant recovery of mechanical properties, such as storage modulus, loss modulus, and damping tan δ (at 0.025 strain) by 60-110% after being stressed to the point of gel failure. Furthermore, the dry NC gel showed exceptional thermal and mechanical stability during a heating and cooling cycle between 25 and 110 °C, with only slightly decreases followed by at least 30% increases in both moduli, while tan δ remained nearly unchanged. The NC gel in dry state could repeatedly adhere to various surfaces such as steel, glass, plastic, etc., and detach from the surface without being broken and leaving little contamination behind. This unique adhesive characteristic was characterized by high storage modulus, loss modulus (kPa), and tan δ (>0.6) corresponding to high cohesive, adhesive, and tacking properties of pressure-sensitive adhesives (PSAs). Finally, a reversible network structure formed by PEO interpenetrating within 3-dimentional (3-D) silica network was proposed to be responsible for the dynamic restructuring and the unique adhesive behaviors observed in the NC gel, and the 3-D network structure was investigated by XRD, FTIR, and DSC measurements. For this 3-D network structure, we suggest that the flexibility of PEO could allow PEO side chains to contact with various surfaces by either PEO segments or methoxy end groups via weak physical interactions, such as van der Waals interactions or hydrogen bonding, whereas the reversible network structure contributes to the recovery of strength and shape after the gel failure.

An amphiphilic-like fluoroalkyl modified SiO2 nanoparticle@Nafion proton exchange membrane with excellent fuel cell performance

A new route to enhance the cell performance of a Nafion proton exchange membrane is provided by incorporating fluoroalkyl modified SiO2 nanoparticles. In favor of the achieved amphiphilic-like surface characteristics of SiO2 nanoparticles, the so-formed nanocomposite membrane exhibited great enhancement of single cell performance at 80 °C: ~34% increase in output power relative to the Nafion reference and a superior maximum output power as high as 579.6 mW cm(-2).

Lightweight flexible carbon nanotube/polyaniline films with outstanding EMI shielding properties

Lightweight flexible carbon nanotube (CNT)/polyaniline (PANI) free standing films having the highest reported specific shielding effectiveness (SSE) of 7.5 × 104 dB cm2 g−1 were developed using amine-functionalized CNTs, which could be attributed to the high conductivity (3009 S cm−1) and low density of the amine-functionalized CNT/PANI composites. The mechanism behind the high conductivity is also studied.